1. Field of the Invention
The present invention relates to a power control method, a power control device, and an image forming apparatus. The present invention is used for power control in an image forming apparatus using electrophotography, e.g., a copier, a printer, a facsimile machine, or a combination machine thereof. In particular, the present invention is used for power control on a fixing heater of an image forming apparatus in an environment in which, for example, a frequency or a voltage of an AC power supply varies substantially, or, in an electrically noisy environment.
2. Description of the Related Art
An image forming apparatus employing electrophotography is usually provided with a heater to fuse, by heat, a toner image formed on paper to fix the same thereon. In recent years, such a fixing heater has tended to consume a large amount of power. This causes a power supply voltage to drop due to a large amount of inrush currents, which has the adverse effect of flickering of fluorescent lights in the power system where the heater is provided. Further, the inrush currents probably damage a switching device such as a transistor or a triac to control the driving of the heater.
In order to solve such problems, through up-regulation is performed to gradually increase a conduction current in an early stage where the heater is energized. In the through up-regulation, a phase angle at which a switching device is turned ON in each half cycle of AC is controlled to gradually increase a conduction angle φ at which the switching device is turned ON from 0 to π. This can increase the temperature of the heater as quickly as possible, and further, minimize inrush currents. Further, through down-regulation is performed, as necessary, to gradually reduce the conduction current at a time when supplying power to the heater is finished. Such phase control makes it possible to control the temperature of the heater in a stable manner.
Meanwhile, in a case where an image forming apparatus is located in an undesirable power supply environment, e.g., a case where the apparatus is located in an environment where a power distribution line has high impedance, voltage drop of voltage inputted to the image forming apparatus occurs in the form of noise at a time when the heater is turned ON during the phase control.
In such a case, a zero-crossing signal used for the phase control is sometimes outputted not at a proper time of a zero-crossing point but at an improper time. Thereby, the phase control is performed synchronously with an improper zero-crossing signal, so that the proper phase control cannot be performed in a stable manner. In such a case, since devices are prone to malfunction, it is sometimes determined that an error occurs in the power supply and the devices are controlled to stop.
In relation to such problems, a technology is proposed in Japanese Laid-open Patent Publication No. 2007-064892. According to the technology, when the zero crossing pulse width of an input AC power supply is less than a predetermined range, it is determined that a failure occurs from the power supply noise. When the zero crossing pulse width is the predetermined range or larger, it is determined that a failure occurs from power supply interruption.
Another technology is disclosed in Japanese Laid-open Patent Publication No. 2009-204638. According to the technology, in order to prevent malfunctions of a heater by ignoring the interrupt of a zero-crossing generated due to noise, a control section ignores the rise in the zero-crossing signal during setting time T from the rise of the heater control signal.
However, according to the foregoing technologies disclosed in Japanese Laid-open Patent Publication No. 2007-064892 and No. 2009-204638, if power supply noise is present in a predetermined range of a zero-crossing pulse, and the zero-crossing signal is erroneously deemed as a true zero-crossing signal, or, alternatively, if a true zero-crossing signal overlaps power supply noise, then the true zero-crossing signal cannot be detected.
As discussed earlier, an AC power supply may be unstable depending on an environment in which an image forming apparatus is installed. This causes a voltage and a frequency to vary, so that the waveform of a zero-crossing signal varies and power supply noise is caused in some cases. In such a case, even if a predetermined period is determined as a period during which a false zero-crossing signal is generated, the period probably changes with the variation in voltage and frequency. As a result, detecting a normal zero-crossing signal is impossible.